Page 148 - New Trends in Eco efficient and Recycled Concrete
P. 148
122 New Trends in Eco-efficient and Recycled Concrete
removed from the bottom of the decanter tanks as sewage sludge (SS). In many
cases, the addition of chemical reagents (e.g., polyelectrolytes, iron or aluminium
salts) facilitates and accelerates the settling process.
There are several types of secondary treatments (biological), although the most
common involve the digestion of organic matter, either by aerobic or by anaerobic
processes, both based on the action of bacteria. The process results in water with a
low organic content (lowering of the COD and BOD values below established lim-
its) and mud with a high-water content.
Tertiary treatments try to eliminate some compounds, or groups of specific com-
pounds, such as those related to nitrogen (nitrates and nitrites) and those related to
phosphorus (phosphates). There are also processes for the removal of some soluble
organic compounds, such as pharmaceuticals.
Both types of SS generated in primary treatments and secondary treatments are
liquid, or semi-solid liquid, and require water removal processes (dewatering by
centrifugation, plate pressing or bed drying, etc.) so that the generated SS can be
transported and handled in a convenient way. The solid content ranges in 0.25%
12% by weight (Metcalf, 1991). In some cases, SS is anaerobically digested and
then the chemical composition changes in terms of total solid content and volatile
compounds. The processes for humidity reduction produce a sludge with humidity
ranging from 80% to 20%, depending on the technology used.
All of these SS contain, in addition to water, a large percentage of diverse
organic matter and pathogens. They also contain inorganic components. In these
inorganic components, we can find various types of salts (such as sulphates and
phosphates) and minerals, such as quartz, feldspars and also clays, among other
substances. Typical composition ranges for untreated and digested sludge are
reported in Table 5.1 (Fytili and Zabaniotou, 2008).
Table 5.1 Range of chemical compositions for untreated and digested sludges
Parameter Untreated sludge Digested sludge
Total dry solids (TS, %) 2 8 6 12
Volatile solids (%) 60 80 30 60
Grease and fats (ether soluble, %) 6 30 5 20
Proteins (%) 20 30 15 20
Nitrogen (N, %) 1.5 4.0 1.6 6.0
Phosphorous (P 2 O 5 , %) 0.8 2.8 1.5 4.0
Potash (K 2 O, %) 0 1 0 3
Cellulose (%) 8 15 8.15
Iron (not as sulphide, %) 2 4 3 8
Silica (SiO 2 ,%) 15 20 10 20
Note: All parameters, except total dry solids (TS), are given as a weight percentage of TS.
Data taken from Fytili, D., Zabaniotou, A., 2008. Utilization of sewage sludge in EU application of old and new
methods—a review. Renewable Sustainable Energy Rev. 12, 116 140. Available from: https://doi.org/10.1016/j.
rser.2006.05.014.